Method of inspecting the welding of a sealed closure plug of a nuclear reactor fuel element

ABSTRACT

Before welding, the plug and the sleeve of the fuel rod being in the welding position at the welding station, the sleeve and the plug of the fuel rod are rotated about their common axis with the aid of rotation means, images are taken along the periphery of the fuel element to obtain digitized images, the digitized images are analyzed to determine the position of the joint plane between the sleeve and the plug of the fuel rod, and the rotation of the fuel element is verified. It is deduced whether it is possible to perform the welding or not. If welding is performed, images are taken of the fuel element along the periphery of the fuel element, in the vicinity of a joint line between the sleeve and the plug, to obtain digitized images which are analyzed to check the conformance of a weld made along the joint line.

FIELD OF THE INVENTION

The invention relates to a method of inspecting the welding of a sealedclosure plug of a nuclear reactor fuel element.

BACKGROUND INFORMATION

Nuclear reactors, such as those cooled by pressurised water, include acore made up of fuel assemblies in which energy in the form of heat isproduced during the operation of the reactor.

Each of the fuel assemblies is generally constituted of a bundle ofparallel fuel elements retained in the framework of the fuel assembly.Each of the fuel elements includes a tubular sleeve made from a materialthat is poor at absorbing neutrons, such as a zirconium alloy, in whichare stacked nuclear fuel pellets, for example sintered pellets ofuranium oxide (UO₂). The tubular sleeve is closed at both ends by a plugwhich has a cylindrical part which is inserted coaxially into an endpart of the sleeve. The plug and the sleeve are then welded togetheralong a circular line in a plane substantially perpendicular to the axisof the sleeve and the plug.

The fabrication of fuel elements requires many successive operations offilling the sleeve with the fuel pellets, fitting and welding the plugsand introducing a pressurised inert gas such as helium into the sleevesealed by the plugs. Many inspection operations must be carried outduring all steps of fuel element fabrication so that fuel elements areobtained with zero defects.

In particular, the welds between the plugs and the ends of the sleevemust be rigorously inspected.

The plugs are welded in a welding station including a sealed chambercontaining an inert gas atmosphere into which part of the end of thesleeve with a plug inserted in it is introduced. The sleeve and the plugare coupled to a system for rotating them about their common axis insidethe sealed welding chamber. The welding is effected by melting parts ofthe sleeve and the plug disposed one against the other in a joint planeperpendicular to the axis of the fuel element. A welding system such asa TIG welding torch or a laser beam welding head is used to melt theplug and the sleeve in the welding area. The electrode of the TIGwelding torch or the beam from the laser welding head is disposed in theplane of the joint and substantially perpendicular to the axis of thefuel element. The plug has a shoulder between the cylindrical partinserted into the sleeve and a larger diameter part which remainsoutside the sleeve. Because of the accuracy with which the end surfacesof the sleeve and the plug are machined, the contact between the endpart of the sleeve and the shoulder on the plug cannot be perfect and inpractice the joint plane consists of the edge surface at the end of thesleeve and the surface of the shoulder on the plug, which are separatedby a small annular interstice. When the weld is effected by melting thesleeve and the plug in the joint plane, the interstice must be closedand entirely filled with molten metal to obtain a perfect seal of thejoint between the plug and the sleeve of the fuel element.

To obtain a proper weld it is necessary for the joint plane as justdefined to be perfectly located relative to the welding axis, which isthe axis of the electrode in the case of TIG welding or the axis of thebeam in the case of laser welding.

After the weld is executed, it is necessary to verify that the plug andthe sleeve were melted all around the circular joint line, inside thejoint plane, and that there is no discontinuity in the weld, which couldlead to a defective seal.

What is more, to avoid reducing the productivity of the production line,the welds between the plugs and the sleeves of the fuel elements mustalso be inspected in such a way that the fuel element fabrication timeis not unduly increased. It is also desirable to allow for producing afast diagnosis indicating whether the fuel element can be accepted.

Optical inspection of fuel element welds based on digitized imagesobtained by a scanning camera coupled to an image digitizer system hasbeen proposed, for example in U.S. Pat. No. 5,602,885.

From each digitized image, a matrix is obtained of values of thereflective power of each pixel of the image, arranged in columns androws of the image. A mean value of the reflective power of the pixels ofthe image is calculated and compared to the reflective power of eachpixel. If the values for a particular number of adjoining pixels,corresponding to an area of the weld having the minimum size of a defectthat can be detected, depart from the mean value by an excessive amount,the presence of an unacceptable welding defect is diagnosed.

A method of the above kind is used in an inspection station separatefrom the welding station, which means that it is necessary to pass thewelded fuel elements from the welding station to the inspection station.The handling time between the welding station and the inspection stationand the time needed to perform the inspection are therefore added to thefuel element fabrication time.

It is also necessary to provide rotational control means to inspect theweld by taking images along the circular weld line.

What is more, the above method provides no way of monitoring thepositions of the plug and the sleeve before welding or of effectivelytesting the conformance of the weld, regardless of the type of weldingemployed.

SUMMARY

The object of the invention is therefore to propose a method ofinspecting the welding of a sealed closure plug of a fuel element for anuclear reactor, the fuel element including a tubular sleeve enclosing aplurality of nuclear fuel pellets stacked in the axial direction of thesleeve and two sealed closure plugs having a cylindrical part insertedcoaxially into an axial end part of the sleeve, a plug being welded in awelding station by melting the sleeve and the plug along a circular linein a joint plane perpendicular to the axis of the sleeve and the plug bya welding arrangement directed radially relative to the circular line inthe joint plane of the sleeve and the plug, which are coupled to arotation arrangement for rotating them about their common axis,inspection being effected by processing digitized optical images ofareas of the fuel element adjoining the circular joint line anddistributed along the periphery of the fuel element, this methodchecking that the plug is fitted correctly to the sleeve of the fuelelement before welding, and checking the conformance of the weld, at thewelding station, and in masked time, during the operation of welding thefuel element.

To this end before welding, the plug and the sleeve being in the weldingposition at the welding station, the sleeve and the plug are rotatedabout their common axis with the aid of the rotation arrangement, imagesare taken along the periphery of the fuel element to obtain digitizedimages, the digitized images are analysed to determine the position ofthe joint plane, and the rotation of the fuel element is verified, it isdeduced whether it is possible to perform the welding, and if thewelding is performed, after welding the plug to the sleeve of the fuelelement, images are taken of the fuel element in position at the weldingstation, along the periphery of the fuel element, in the vicinity of thejoint line, to obtain digitized images, and the digitized images areanalysed to check the conformance of a weld made along the joint line.

BRIEF DESCRIPTION OF THE DRAWINGS

To explain the invention clearly, the implementation of the inspectionmethod according to the invention will now be described by way ofexample and with reference to the accompanying drawings, in the case ofTIG welding and in the case of laser welding of a plug to the sleeve ofa fuel element of a pressurised water nuclear reactor.

FIG. 1 is a partial view in axial section of the end of a fuel elementbefore welding the closure plug.

FIG. 2 is a partial view in axial section of the end of a fuel elementafter welding the closure plug.

FIG. 3 is a diagrammatic view of a TIG welding station for fuel elementplugs and control means used to implement the method according to theinvention.

FIG. 4 is a diagrammatic view of a laser welding station for fuelelement plugs and control arrangement used to implement the methodaccording to the invention.

FIG. 5 is an image shown on the screen of the inspection system duringexecution of the method according to the invention in the case of TIGwelding. FIG. 5 also relates to checking the position of the joint planebefore welding. FIG. 5 further relates to checking the position of thejoint plane before welding.

FIG. 6 is an image shown on the screen of the inspection system duringexecution of the method according to the invention in the case of TIGwelding. FIG. 6 also relates to checking the position of the joint planebefore welding. FIG. 6 further relates to checking the position of thejoint plane before welding.

FIG. 7 is an image shown on the screen of the inspection system duringexecution of the method according to the invention in the case of TIGwelding. FIGS. 7 also relates to inspecting the weld.

FIG. 8 is an image shown on the screen of the inspection system duringexecution of the method according to the invention in the case of TIGwelding. FIG. 8 relates to inspecting the weld.

FIG. 9 is a diagram showing the grey level along a line of the imagepassing through the joint plane.

FIG. 10 is an image shown on the screen of the inspection system whenthe method according to the invention is used to inspect a laser weld.FIG. 10 also relates to locating the joint plane.

FIG. 11 is an image shown on the screen of the inspection system whenthe method according to the invention is used to inspect a laser weld.FIG. 11 also relates to locating the joint plane.

FIG. 12 is an image shown on the screen of the inspection system whenthe method according to the invention is used to inspect a laser weld.FIG. 12 shows a pulsed laser weld.

FIG. 13 is a diagram referred to in the description of the method ofinspecting a pulsed laser weld. FIG. 13 is also a diagram showing thegrey level of points along a column of the image obtained duringinspection inside and outside the weld area.

FIG. 14 is a diagram referred to in the description of the method ofinspecting a pulsed laser weld. FIG. 14 is also a diagram showing thetransitions along columns of the image between grey level minima andmaxima which conform to the period of the welding pulses.

FIG. 15 is a diagram referred to in the description of the method ofinspecting a pulsed laser weld. FIG. 15 is also a diagram deduced fromthe diagram of FIG. 14 by filtering to locate the edges of the weld.

FIG. 16 is an image shown on the screen of the inspection system wheninspecting laser welds.

FIG. 17 is an image shown on the screen of the inspection system wheninspecting laser welds.

DETAILED DESCRIPTION

FIGS. 1 and 2 show the end part of a fuel element 1 for a pressurisedwater nuclear reactor.

The fuel element 1 includes in particular a zirconium alloy tubularsleeve 2 which contains fuel pellets 3 and which is closed at its endsby plugs such as the plug 4 closing the end of the sleeve 2 shown inFIGS. 1 and 2.

The plugs 4 are generally made of zirconium alloy and have a part 4 awhich is inserted virtually without clearance into the end part of thebore of the sleeve 2.

As shown in FIG. 1, the plug 4 is inserted into the sleeve 2 in acoaxial arrangement, the axis of the plug and the axis of the sleeve 2coinciding with the longitudinal axis 5 of the fuel element.

The plug 4 has a shoulder perpendicular to the axis 5 between itssmaller diameter cylindrical part 4 a which is inserted into the sleeve2 and a part of the plug 4 which remains outside the sleeve. After theplug is inserted in the closure position, a very small annularinterstice 6 remains between the shoulder on the plug and the end of thesleeve.

After the sleeve is welded, as shown in FIG. 2, the weld 7 fills andcloses the interstice 6, joins the sleeve 2 to the plug 4, and seals thejoint between the sleeve and the plug.

The welding is performed by rotating the sleeve 2 into which the plug 4is inserted inside a welding station about the axis 5 common to thesleeve and the plug and melting the end of the sleeve 2 and a portion ofthe plug 4 using a welding device disposed radially relative to thecircular joint line between the sleeve and the plug in a joint planeperpendicular to the axis 5 common to the sleeve 2 and the plug 4.

The expression “joint plane” refers to the area between two planesperpendicular to the axis 5, one plane containing the end of the sleeve2 and the other plane containing the shoulder on the plug 4. Thus thecircular joint line is in fact an annular area extending along theinterstice 6 and in which the weld 7 is formed during welding.

FIG. 3 illustrates a welding station of a first embodiment of theinvention for welding plugs to the end parts of fuel elements by a TIGwelding process, i.e. an electrical arc welding process carried out inan inert gas atmosphere using a tungsten electrode. FIG. 3 alsoillustrates an inspection system for implementing the method accordingto the invention.

The welding station 8 includes a closed and sealed welding chamber 9which contains an inert gas such as argon and inside which plugs arewelded to the end parts of fuel element sleeves.

The chamber 9 has on one of its lateral faces a sealed bushing 10 bywhich an end part of a fuel element 1 analogous to the end part of thefuel element shown in FIG. 1 passes through the wall of the chamber.

A vertical TIG welding torch 11 is mounted on the top face of thechamber 9 and includes a vertical tungsten welding electrode 12 whichpasses through the torch 11 and therefore enters the chamber, the endpart of the electrode having an axis constituting the welding axis whichis disposed in the joint plane between the plug and the sleeve of thefuel element 1 and in a direction perpendicular to the axis of the fuelelement, i.e. a radial direction relative to the circular joint linebetween the plug and the sleeve of the fuel element 1.

To execute the welding, the electrode is fed with an electrical currentwith a potential difference between the electrode and the sleeve and theplug of the fuel element such that an electrical arc is struck betweenthe joint area between the plug and the sleeve and the tip of theelectrode, which is inside the sealed enclosure and at a small distancefrom the joint area.

A first lateral face of the welding chamber 9 perpendicular to the facecarrying the sealed bushing 10 for the fuel element 1 includes atransparent porthole 13 through which light is directed towards thejoint area of the fuel element by an illumination device 14. This firstlateral face of the chamber 9 is referred to as the front face and theillumination device 14 is referred to as the front illumination device.A second or rear face of the chamber 9 carries a porthole and a rear(backlight) illumination device 15 for illuminating the joint area ofthe fuel element 1.

A digital camera 16 including an optical system 16 a and a digitizermodule is connected to a microcomputer 17 via an image acquisition card.The microcomputer 17 also includes a digital input/output card enablingthe microcomputer to communicate with an automatic control system of theTIG welding equipment. In particular, after checking the position of thejoint plane relative to the welding axis, the microcomputer communicatesto the welding control system an instruction authorising or prohibitingwelding, depending on the result of the check on the position of thejoint plane. Similarly, a verdict is transmitted to the control systemafter inspecting the weld.

Images of the joint area of the fuel element 1 reach the optical part 16a of the digital camera 16 via the front porthole 13 and are digitizedbefore they are transmitted to the microcomputer 17 via the imageacquisition card.

The microcomputer 17 has a screen 18 on which the joint area of the fuelelement and inspection results can be displayed.

As indicated above, a first step of the process of inspecting the weldin accordance with the invention is to check the position of the jointplane between the plug and the sleeve of the fuel element beforewelding.

In the case of TIG welding, the position of the joint plane isdetermined relative to the axis of the tungsten electrode, the width ofthe joint plane in the axial direction of the fuel element, and thedistance between the tip of the electrode and the joint area of the fuelelement, and a diagnosis relating to the position of the joint arearelative to the electrode is produced.

The joint plane is inspected while sweeping the joint area of the fuelelement inside the welding chamber with an inert gas and rotating thefuel element about its axis inside the welding chamber by a rotationsystem of the welding station.

In this way it is possible to locate and determine the position of thejoint plane in a plurality of areas distributed around the periphery ofthe joint area of the fuel element.

For example, eight successive operations can be carried out to locateand determine the position of the joint plane in eight areas around theperiphery of the fuel element, to determine the conformance of theposition of the joint plane relative to the electrode. In this case, inorder to issue a verdict concerning the conformance of positioning, itis possible to choose a number of conform searches, i.e. searches whichreveal no defects in respect of the position or the width of the jointplane over the total number of joint plane location and determinationoperations effected. In the case of eight operations to locate anddetermine the position of the joint plane, for example, the number ofconform searches chosen is five.

This procedure also verifies that the fuel element has rotated correctlybetween the successive operations to locate and determine the positionof the joint plane.

The rotation of the fuel element is deemed to be non-conform if all thepositions of the fuel element determined by the successive joint planelocation and determination operations are identical.

Because the joint plane is in practice an area delimited between twoplanes perpendicular to the axis of the fuel element, the position ofthe joint plane is determined by the distance in the axial directionbetween at least one of the two planes and the axis of the electrode,the latter constituting the welding axis.

For example, in the case of the location process described below, thedistance between the left-hand edge of the joint plane in the image andthe axis of the tungsten electrode in the axial direction is determined.

The width of the joint plane corresponds to the axial distance betweenthe end of the sleeve and the shoulder on the plug, i.e. the width ofthe interstice 6, which may vary along the circular joint line.

The first step is to carry out measurements relating to the electrode 12on the digitized images, as shown in FIGS. 5 and 6.

The electrode is first located along an electrode search line 19 whichis perpendicular to the joint plane, i.e. which is horizontal in theimage displayed on the screen, as may be seen in FIGS. 5 and 6. A searchis conducted along the search line 19 to find grey level transitionscorresponding to the edges of the electrode 12. If the search does notfind the electrode, a fault signal is issued.

The electrode search line 19 is automatically centred on a referenceline 20. The search line and the reference line 20 are positionedvisually if the TIG welding station has been set up correctly.

Following location of the electrode, the welding axis can be determined,and in the case of TIG welding corresponds to the axis of the electrodeas found previously. A search for the joint plane 22, which correspondsto the interstice 6 between the fuel element and the plug, is thencarried out (see FIGS. 5 and 6).

To this end, the fuel element and the plug 4 are illuminated in amaximum illumination area 21 inside the welding chamber that can be seenin FIGS. 5 and 6. The joint plane is located along search lines such asthe line 19′ in the illuminated area 21 shown in FIGS. 5 and 6.

The reference line 20, which is vertical in the image, is positioned atthe left-hand edge of a theoretical joint plane, and the search linesare centred on the reference line; the position of the real joint planeis determined by the horizontal distance between the left-hand edge ofthe joint and the axis of the electrode previously determined. The widthof the joint plane is determined by the horizontal distance between theleft-hand edge and the right-hand edge of the joint.

The right-hand edge and the left-hand edge of the joint plane are lookedfor on search lines 19′ centred on the reference line 20, using aprocessing method described below with reference to FIG. 9.

The distance between the tip of the electrode 12 and the fuel element isalso measured, along the axis of the electrode previously found, i.e. ina vertical direction in the image, by measuring the distance betweengrey level transitions detected on that axis. The exit of the electrodeat the tip is reflected in a black-white transition in the image and itsentry into the fuel element by a white-black transition. The entry/exitdistance in pixels is measured along a vertical column.

FIG. 9 illustrates the search for the joint plane and is a diagramgiving the grey levels of pixels along a mean search line which isestablished during previous processing of the image.

A parameter N entered into the processing system corresponds to thenumber of rows above and the number of rows below the position of thesearch line, such as the line 19′, and provides a mean of the greylevels over the corresponding 2×N rows. The parameter N is referred toas the mean number of rows. The curve 23 in FIG. 9 represents the greylevels along the mean line, with the points on the line plotted on theabscissa axis. There are 245 points for the whole of the line, forexample.

The minimum and maximum values on the curve 23 representing the greylevels along the mean line are determined.

A threshold value is calculated and is equal to half the sum of themaximum value and the minimum value determined previously. A straightline segment 24 is drawn parallel to the abscissa axis and correspondsto this threshold value.

A search for the left-hand edge of the joint plane is then conductedbased on the grey levels of the digitized image pixels, starting fromthe left-hand end of the mean search line. The left-hand edge is deemedto have been found as soon as three consecutive points are detectedbelow the threshold shown by the straight line segment 24. The left-handedge 25 of the joint plane is determined in this way. The position ofthe left-hand edge relative to the edge of the image is determined andthe distance between the left-hand edge and the axis of the electrodefound previously is calculated.

The position of the right-hand edge of the joint plane is thendetermined by considering the grey levels of the successive pixels fromthe right-hand end of the mean search line and comparing those greylevels to the threshold value represented by the straight line segment24.

The right-hand edge 26 of the joint plane is deemed to have been reachedas soon as three points are detected under the threshold.

The width of the joint between the left-hand edge 25 and the right-handedge 26 can then be determined.

The values representative of the position of the joint, i.e. thedistance between the left-hand edge of the joint and the axis of theelectrode, and the width of the joint, are then compared to thresholdsdefined by the following parameters: left-hand electrode positiontolerance, right-hand electrode position tolerance and maximum width ofjoint plane.

The results of the comparison are shown on the screen of themicrocomputer 17.

If at least one threshold value is exceeded, a diagnosis is issued andan instruction is transmitted to the control system so that welding isnot performed.

FIG. 5 illustrates the image appearing on the screen in the case of asatisfactory search for the joint plane 22, which is located exactlyalong the reference line 20 coincident with the axis of the electrode.

FIG. 6 illustrates the image appearing on the screen in the case of anon-conform joint plane, the joint plane 22 being offset to the leftrelative to the position of the reference line 20.

Similarly, unfavourable diagnoses can be issued if the distance betweenthe tip of the electrode and the fuel element is outside the specifiedrange or if the width of the joint plane is above a threshold value.

The search for the joint plane and the determination of its position andwidth can be effected in a plurality of areas at the periphery of thefuel element, which is rotated about its axis.

Continuous inspection of the joint plane is also possible by rotatingthe fuel element and taking successive images, each of which isprocessed before the next image is taken.

After carrying out the TIG welding, during the weld cooling phase, theweld is inspected in order to issue a final verdict on the quality ofthe weld, which is transmitted to the welding station control system.

The weld is inspected by a process identical to the joint plane searchprocess, as described above. The presence of a defect is logged if thejoint plane is detected.

If two consecutive defects are detected in the weld, inspection isstopped and the weld is declared defective in one area of the fuelelement. Inspection continues as long as two consecutive defects are notdetected. At the end of inspection the final verdict is sent to thewelding station control system.

The weld is inspected in accordance with the following sequence:continuous acquisition of images of the weld, the next image beingacquired during the processing of the previous image, location of theweld in each image; if N consecutive weld integrity defects are detected(generally two consecutive defects), inspection is stopped and the weldis declared defective; N is the reject threshold parameter, display ofresults.

FIG. 7 illustrates a conform weld in a strongly illuminated area 21 ofthe fuel element 1, the weld being totally invisible in the image. Nopart of the joint plane appears in the image.

FIG. 8 illustrates a defective weld in a strongly illuminated area 21 ofthe fuel element 1. The joint plane 22 has been detected in this area.

FIG. 4 illustrates a station for welding plugs to fuel elements using alaser beam and a system for inspecting the weld using a method accordingto the invention.

The welding station 28 includes a welding chamber 29 into which the endof the fuel element 1 including the plug is inserted through a sealedbushing device 30 passing through one lateral face of the chamber 29. Anadjustable abutment device 31 fixed to the lateral face opposite theface including the bushing device 30 for the fuel element 1 adjusts theposition of the joint area of the plug and the sleeve of the fuelelement 1 relative to the laser beam welding device.

The welding station 8 includes in particular an optical system 32 whichincludes a mirror for deflecting and focussing the laser beam. Theoptical system 32 is connected to the laser generator by an opticalfibre 27 connected to a collimator 33.

The optics 34 of a digital camera 35 and an illumination system 36 arefixed to the top face of an enclosure of the optical system 32.

The digital camera 35 is connected to a microcomputer 37 including adisplay screen 38.

As in the case of TIG welding, inspection of the joint plane beforewelding is followed by inspection of the weld if the joint plane isdeemed to be conform.

The position of the joint plane is located and determined by a methodsimilar to the method used in the case of TIG welding.

The method is therefore not described again.

However, the reference line relative to which the position of the edgesof the joint plane is determined is defined and determined differentlyin the case of laser beam welding than in the case of TIG welding.

In the case of TIG welding, the reference line serves only to supportthe search lines 19 and 19′. The position of the joint plane isdetermined relative to the axis of the electrode, which is detected byimage processing.

In the case of laser welding, a fixed reference is used in the form of avertical line on the screen which constitutes a marker relative to whichthe position of the edges of the joint plane is determined.

After setting up the laser welding station, the laser is fired onto thesurface of the fuel element and the reference line is chosen as thevertical line of the image passing through the trace of the laser beam.The reference line therefore corresponds to the welding axis. Theposition of the joint plane relative to the reference line and the widthof the joint plane are determined and rotation of the fuel element isverified.

FIG. 10 illustrates, in a strongly illuminated area 41 of the fuelelement 1, the reference line 40, a joint plane search line 39 and thejoint plane 40 and 42. The left-hand edge of the joint plane iscoincident with the reference line 40 and the conforming width of thejoint plane.

FIG. 11 illustrates a non-conform joint plane 42 offset to the rightrelative to the reference line 40. If the joint plane is conform, afavourable diagnosis is issued and a signal authorising welding is sentto the control system.

The welding is effected by a pulsed laser beam while the fuel element isrotating.

In FIG. 12, the weld 45 made by the pulsed laser beam and centred on thereference line 40 includes successive waves 44 of substantially circularshape each corresponding to one firing of the laser. The distancebetween the waves 44 in the vertical direction of the image,representing the circumferential direction of rotational movement of thefuel element, corresponds to the movement between two successive pulsesof the laser beam.

Digitized images are acquired continuously, the next image beingacquired during the processing of the previous image.

The left-hand and right-hand edges of the weld are located and theirpositions relative to the reference line 40 are determined. The width ofthe weld is also determined, from the positions of the edges of theweld.

Because there is a correlation between the width of a laser beam weldand the depth of penetration of the weld, it is possible to determinefrom the width of the weld how deep the weld has penetrated into thesleeve and the plug of the fuel element.

FIG. 13 is a diagram showing the grey levels, on a scale from black towhite, of the pixels of an image of the weld in a first column of theimage (i.e. along a vertical line of the image, corresponding to aperipheral circular line on the fuel element in a plane perpendicular tothe axis of the fuel element) inside the weld and in a second column ofthe image outside the weld.

The curve 46 corresponds to a column of the image outside the weld andthe curve 47 to a column of the image inside the weld.

The successive waves of the weld are reflected in peaks and troughsindicated by the respective arrows 48 and 49.

The arrow 50 shows the distance the fuel element moves in a time periodcorresponding to the period of the laser beam pulses.

Diagrams similar to the FIG. 13 diagram are produced for each column ofthe image of the weld and each diagram obtained, similar to the FIG. 13diagram, is searched for pairs of transitions between a trough and apeak which have a period consistent with the period of the weldingpulses.

The number of transitions consistent with the period of the weldingpulses is logged for each column of the weld and for the areas adjoiningthe lateral edges of the weld. FIG. 14 shows the number of transitionsas a function of the columns of the image.

FIG. 14 shows the reference line 40 and the peaks corresponding to thenumber of transitions consistent with the period of the welding pulsesfor each column of the image.

The maximum number of transitions in the portion of the diagram to theleft of the reference line 40 is determined, and is referred to as theleft-hand maximum.

Similarly, the maximum number of transitions in the portion of thediagram to the right of the reference line is determined, and isreferred to as the right-hand maximum.

The central mean of the transitions between the left-hand maximum andthe right-hand maximum is also determined and is shown by the straightline segment 51 parallel to the abscissa axis.

A weld is deemed to have been executed on the fuel element if theleft-hand maximum and the right-hand maximum are above a particularlimit, for example 4, and the central mean is above a particular limit,for example 3.

A central mean above a particular limit indicates the presence of a weldin the central area.

If the conditions relating to the left-hand maximum and the right-handmaximum and/or the central mean are not complied with, a search for thejoint plane is conducted. If the joint plane is not found, inspectioncontinues.

If the joint plane is found, a weld integrity defect diagnosis isissued.

The FIG. 15 diagram is produced by filtering the FIG. 14 diagram, andshows only transitions relating to the edges of the weld, excluding thecentral part.

A search for the left-hand edge of the weld 45 and a search for theright-hand edge are conducted successively.

To this end, a right-hand threshold and a left-hand threshold arecalculated to provide a weld edge search criterion.

The right-hand threshold is defined by the following equation, in whichx is a parameter referred to as the right-hand weld search threshold:

RH threshold=RH min+x% of (RH max−RH min).

Similarly, the left-hand threshold is defined by the following equation,in which y is a parameter referred to as the left-hand weld searchthreshold:

LH threshold=LH min+y% of (LH max−LH min).

The values of x and y are determined according to the fuel elementillumination conditions.

If one of the above parameters is not satisfactory for precisedetermination of the edges, it is adjusted by entering a new value ofthe parameter into the processing software.

The search for the left-hand edge and the right-hand edge is conductedfrom threshold values represented by straight line segments 52 and 53 inFIG. 15, for example by searching for a particular number of pixelsbelow the threshold in a first direction from a maximum followed by apixel above the threshold in the second direction. For example, fivepoints are searched for under the threshold in one direction and thenone point above the threshold in the second direction. This produces theposition of the left-hand edge and the right-hand edge along the columnsof the image, represented in FIG. 15 in the form of vertical segments 55and 56, and the position and the width of the weld 45 are determined inthis way from the number of columns between the left-hand and right-handedges and between those edges and the edge of the window for determiningtheir positions; these values are compared to threshold valuesconstituting the following parameters of the processing system:left-hand marker, position tolerance and right-hand marker positiontolerance.

The width is also compared to a threshold value constituting a lack ofweld minimum width parameter.

If three consecutive widths less than the minimum width are encounteredwhen the successive images of the welds are processed, the absence of aweld is deduced and inspection is stopped. The weld is declareddefective. If three consecutive widths less than the minimum width arenot encountered, inspection continues and at the end of inspection thepositions and widths of the weld are averaged and then compared to theposition limits (right-hand position tolerance, left-hand positiontolerance) and width limits (minimum weld width) in order to issue afinal verdict that is sent to the control system.

The results are displayed on the screen of the microcomputer.

The rotation of the fuel element is also verified at the end ofinspection by examining the measured width of the weld throughout thelength.

If too few widths depart from the mean value, a diagnosis relating tonon-rotation of the fuel element is issued.

FIGS. 16 and 17 illustrate the image displayed on the screen at the endof inspection. In FIG. 16 (which is substantially the same as FIG. 12),a conforming weld. In FIG. 17, the weld is non-existent and deemed to benon-conforming.

The invention therefore enables the joint plane to be inspected at thewelding station itself, prior to welding, and the weld to be inspectedwith particular reference to the quality and continuity of the weld.

The system can operate in masked time relative to the welding operation.

In the case of laser beam welding, the penetration of the weld ischecked by the correlation between the width of the weld and itspenetration. This parameter is meaningless in the case of TIG welding.

The method according to the invention is used at the welding station,during the welding operation, which avoids all handling operations totransfer the fuel elements between the welding station and an inspectionstation. The verdict relating to the conformance of the weld isavailable as soon as the welding operation is finished.

The information concerning the operation as a whole (welding,positioning and inspection) can be saved on a hard disk for useafterwards in the form of a database.

Finally, the illumination system used by the imaging means is a standardillumination system available off the shelf.

The invention is not strictly limited to the embodiments described.

Thus the method of inspecting the joint plane may be applied to anymethod of welding plugs to nuclear fuel elements.

The digitized images of the weld may be processed by methods other thanthose described in the case of laser beam welding.

Finally, the method according to the invention applies to any nuclearfuel element including sealed closure plugs inserted into end portionsof the sleeve of the fuel element.

What is claimed is:
 1. A method of inspecting welding of a sealedclosure plug of a fuel element for a nuclear reactor, the fuel elementhaving a tubular sleeve enclosing a plurality of nuclear fuel pelletsstacked in an axial direction of the sleeve and two sealed closure plugshaving a cylindrical part inserted coaxially into an axial end part ofthe sleeve, the plug welded in a welding station by melting the sleeveand the plug along a circular line in a joint plane perpendicular to theaxial direction of the sleeve and the plug through welding directedradially relative to the circular line in the joint plane of the sleeveand the plug, which are coupled to a rotation arrangement for rotationabout a common axis, inspection effected by processing digitized opticalimages of areas of the fuel element adjoining the circular line anddistributed along a periphery of the fuel element, the methodcomprising: rotating the sleeve and the plug about the common axis withaid of the rotation arrangement before welding the plug and the sleeve,the plug and the sleeve configured in a welding position at the weldingstation, wherein images are taken along the periphery of the fuelelement to obtain the digitized optical images which are analyzed todetermine a position of the joint plane, and a rotation of the fuelelement is verified; deducing at least one of to perform the welding ornot to perform the welding; and taking images, if the welding isperformed after welding the plug to the sleeve of the fuel element ofthe fuel in the position at the welding station, along a circumferenceof the external surface of the fuel element in a vicinity of the line toobtain the digitized optical images, the digitized optical imagesanalyzed to check a conformance of as weld a long the line.
 2. Themethod according to claim 1, further comprising: determining a referenceline in the digitized optical images prior to welding, the referenceline positioned in a vertical direction of an image corresponding to acircumferential direction of the fuel element, in a plane perpendicularto the axis of the fuel element, and determining grey levels of pixelsof the images along search lines perpendicular to the reference line todetermine a position of two edges of the joint plane in two planesperpendicular to the axis of the fuel element.
 3. The method accordingto claim 2, further comprising: determining a mean search fine from Nadjoining search lines that are determined on a diagram of the greylevels on the mean search line and a detection threshold of the jointplane and comparing the grey levels along the mean search line to thedetection threshold to determine the position of the edges of the jointplane, wherein N is an integer greater than 0 determined by a user. 4.The method according to claim 3, further comprising: determining adistance in a direction of the axis of the fuel element between thereference line and at least one edge of the joint plane.
 5. The methodaccording to claim 2, wherein positions of the edges of a weld aredetermined relative to the reference line.
 6. The method according toclaim 1, wherein during a pulsed laser welding, establishing diagrams ofgrey levels along each column of the image corresponding to peripheraldirections of the fuel element, in planes perpendicular to the axis ofthe fuel element, and, on a curve obtained for each of the columns ofthe image, determining transitions between minima and maxima of thecurve which have a period consistent with a pulsed laser beam period,and, for each of the columns of the image, determining a number of thetransitions having a period equal with the pulsed laser beam period toobtain a curve of a distribution of the transitions along the columns ofthe image, the curve giving the number of transitions per column whichis filtered to obtain a curve of the distribution of the transitions ina vicinity of each edge of the weld, and determining a threshold valueaccording to each edge area of the weld, and determining a condition ofedges of the weld by determining columns in which a number oftransitions are below the threshold.
 7. The method according to claim 6,further comprising: determining a width of the weld between the edges inthe direction of the axis of the fuel element and deducing a depth ofpenetration of the weld therefrom by correlation.
 8. The inspectionmethod according to claim 1, wherein during a laser beam welding where afixed reference line is disposed along a laser beam welding axis,determining the position of the joint plane relative to the fixedreference line.